One of the well known problems associated with heat pumps is that the outdoor coils thereof will, under normal circumstances, have frost accumulate thereon during the heating mode of operation. As the frost thickness increases, then the overall efficiency of the system decreases significantly, and valuable energy is wasted. Accordingly, many schemes have heretofore been proposed for detecting the frost and for taking corrective action for removing the frost from the outdoor coil. Examples of prior art systems include U.S. Pat. Nos. 3,170,304; 3,170,305; and 3,400,553. Other prior art arrangements are disclosed in the co-pending applications of Dale A. Mueller and Stephen L. Serber, Ser. No. 954,141, filed Oct. 24, 1978, U.S. Pat. No. 4,209,994; and Ser. No. 109,742 filed Jan. 4, 1980, U.S. Pat. No. 4,302,947 Application Ser. No. 954,141 discloses an arrangement for using the temperature of the outdoor air to modify the timing function for activation of the defrost mode of operation. The other co-pending application discloses an arrangement for using the temperature of the outdoor coil to vary the duration of the heating interval prior to activation of the defrost mode of operation.
The present invention is an improvement over the arrangements disclosed in said co-pending applications in that it comprises a simplified controller; the controller comprises a variable frequency oscillator which is controlled to oscillate at a frequency which is a non-linear function of the outdoor temperature. The output from the oscillator is applied to a counter which is controlled as a function of the temperature of the outdoor coil; when the counter has attained a preselected number of counts then the defrost mode of operation is commanded. The non-linear function is matched to a preselected schedule of defrost cycles for the heat pump, said schedule in turn being a non-linear function of the temperature of the outdoor air. Alternatively, the variable frequency oscillator is controlled to oscillate at a frequency which is a non-linear function of the outdoor coil temperature.